1. Field
This disclosure relates generally to techniques for calibrating a communication device and, more specifically, to techniques for calibrating a transceiver of a communication device.
2. Related Art
Electronic devices may use various modulation schemes, such as quadrature phase shift keying (QPSK) modulation, for communication. In QPSK, original data that is to be transmitted is separated into two signals (i.e., an in-phase (I) signal and a quadrature (Q) signal). The I signal is typically up-converted by mixing a sine wave of a particular frequency with the I signal and the Q signal is typically up-converted by mixing a cosine wave of the same frequency with the Q signal and, as such, the I and Q signals are ideally phase shifted by ninety degrees. The modulated I and Q signals are then combined following the up-conversion to a higher radio frequency (RF) signal, which is transmitted to and ideally down-converted and demodulated (by a receiver) to provide the original data. Transmitters/receivers may convert a signal to/from an RF signal in a single stage (direct conversion) or in multiple stages. In general, direct conversion architectures may experience relatively large direct current (DC) offsets in the differential I and differential Q baseband signals. The DC offsets, when present, may result in carrier (local oscillator (LO)) leakage and, thus, reduce a range over which a gain of a transmitter can be controlled. The DC offsets may also adversely impact transmitted signal quality, which may be defined in terms of error vector magnitude (EVM) and adjacent channel noise power measurements.
In communication systems such as wideband code division multiple access (WCDMA) communication systems, the range over which transmitter gain is required to be controlled is relatively large. For example, for transmitters operating under third generation (3G) WCDMA specifications, gain control range is required to be at least eighty decibel (dB). To achieve the gain control range for WCDMA, acceptable DC offset (DC imbalance) in the differential I and differential Q baseband signals should generally be less than about one millivolt (0.001 V).
As noted above, transmit LO feed-through may be, for example, attributed to DC imbalance. In a typical case, a DC imbalance at an input of an in-phase/quadrature (I/Q) mixer is converted to RF by a transmit LO (in the I/Q mixer). In this manner, DC imbalance may appear at an output of the I/Q mixer as LO feed-through. According to one known approach for addressing transmit LO feed-through attributable to DC imbalance, a DC offset correction (DCOC) loop has been employed to minimize baseband (BB) transmit path DC offsets at inputs of an I/Q mixer by adjusting digital-to-analog converter (DAC) inputs of a transmitter. Unfortunately, the DCOC loop approach typically requires the BB signal to be set to zero during a calibration sequence prior to transmit and does not remove transmit LO feed-through that is attributable to parasitic coupling directly to an output of an I/Q mixer (which may be a significant component to the overall transmit LO feed-through).
Another known approach to reducing transmit LO feed-through employs an RF detector that is connected (in a transmit path) following a transmitter I/Q mixer. In this case, the RF detector can be used to detect LO feed-through if a BB signal is set to zero during calibration. In general, the RF detector approach facilitates analysis of LO feed-through that is attributable to both DC imbalance (in the transmit path) and parasitic coupling of the transmit LO directly to an output of an I/Q mixer. Unfortunately, when BB gain control is employed (i.e., when varying BB signal levels are applied during actual transmission), an average DC imbalance across inputs of an I/Q mixer changes. In this case, LO feed-through (and the RF output level) varies when BB gain control is employed. As such, the RF detector approach is generally incompatible in situations where variable BB signals are applied and typically cannot discriminate between undesired LO feed-through tones and undesired image tones that are present. Another known approach to reducing LO feed-through has utilized a power detector (e.g., employed to monitor and feedback transmit power levels for output power control) to detect LO feed-through when a BB transmit signal is set to zero. Unfortunately, the power detector approach also does not facilitate minimizing undesired LO feed-through or image tones when a varying BB signal is applied to an input of a transmitter.
An electrical signal may be represented in the time-domain (as a variable that changes with time) or may be represented in the frequency-domain (as energy at specific frequencies). In the time-domain, a sampled digital signal includes a series of data points that correspond to an original physical parameter, e.g., light, sound, temperature, and velocity. In the frequency-domain, a sampled digital signal is represented as discrete frequency components, e.g., sinusoidal waves. A sampled digital signal may be transformed from the time-domain to the frequency-domain using a discrete Fourier transform (DFT). Conversely, a sampled digital signal may be transformed from the frequency-domain to the time-domain using an inverse DFT (IDFT).
As is well known, a DFT is a digital signal processing transformation that is employed in various applications. DFTs and IDFTs facilitate signal processing in the frequency-domain, which can provide efficient convolution integral computation (which is, for example, useful in linear filtering) and signal correlation analysis. As the direct computation of a DFT requires a relatively large number of arithmetic operations, the direct computation of a DFT is typically not employed in real-time applications. Various fast Fourier transform (FFT) algorithms have been created to perform real-time tasks, such as digital filtering, audio processing, and spectral analysis for speech recognition.